Studies are intended to clarify the mechanisms controling the timing of the onset of labor, and particularly the nature of fetal participation. Proposed studies are (1) to evaluate the cytotrophoblastic hypertrophy and hyperplasia that appears in the placenta retained for many weeks after fetectomy in monkeys (2) to measure maternal estrogen levels following fetectomy in monkeys, (3) to attempt to determine the nature of the placental luteotropin in post-fetectomy monkeys by measuring mCG and in somatomammotropin, (4) to study the progesterone binding uterine and plasma proteins in pregnant monkeys and (5) to determine the nature and route of action of a rabbit placental factor maintaining ovarian progesterone production.